To measure temperature by means of infrared emission, thermal (infrared) radiation may be detected by many sensors known in the art. Thermal radiation has electromagnetic nature and thus can be detected either by quantum detectors or by thermal detectors. Quantum detectors, such as photoresistors or photodiodes, require cryogenic cooling to measure relatively low temperatures with high accuracy. On the other hand, thermal detectors, while not as sensitive as quantum, may operate at normal room temperatures. This invention relates to thermal detectors whose most popular application is in noncontact thermometers. One example of such a thermometer is an instant medical ear thermometer which is capable of noncontact measuring temperatures from tympanic membrane and surrounding tissues. Thermal sensors commonly used in infrared thermometers are thermopiles, pyroelectrics, bolometers, and active infrared detectors.
The purpose of an infrared sensor is to generate an electrical signal which is representative of net infrared flux .PHI. existing between the sensor and the object of measurement. The flux depends of two temperatures: the sensor's own surface temperature T.sub.s and that of the object (target) T.sub.b. A relationship between the two temperatures and the flux is governed by Stefan-Boltzmann law: EQU .PHI.=k.epsilon..sub.b .epsilon..sub.s (T.sub.b.sup.4 -T.sub.s.sup.4) (1)
where .epsilon..sub.b and .epsilon..sub.s are emissivities of the target and sensor respectively, and k is a constant. An ultimate goal of a temperature measurement is determination of T.sub.b. It is seen from the above equation that to calculate temperature T.sub.b, one must first determine two variable numbers: a magnitude of infrared flux .PHI. and the sensor's surface temperature T.sub.s.
A surface temperature can be measured by one of many temperature detectors known in the art, like thermistors or thermo-electrics, while measurement of net infrared flux requires an opto-electronic device known as thermal infrared sensor. There are two classes of thermal infrared sensors: passive infrared (PIR) and active far infrared (AFIR). Examples of PIR detectors are pyroelectrics, thermopiles, and bolometers. Measurement of infrared flux by a PIR sensor is not a simple task because PIR sensors with a good speed response are generally fabricated in a form of thin flakes or membranes, whose surface temperature T.sub.s is not only difficult to measure, but that temperature changes upon exposure to a target. Inaccurate determination of sensor's surface temperature T.sub.s results in error in a noncontact temperature calculation. Since sensor's surface temperature in a PIR detector changes upon exposure to a target, to improve response speed of the sensing element, its thermal capacity must be made very small. This imposes quite stringent requirements to a PIR sensor's design and increases cost.
An AFIR sensor, as opposed to PIR, operates at a predetermined (often constant) temperature T.sub.s of a sensing element. The AFIR sensors are based on U.S. Pat. Nos. 4,854,730 and 5,054,936 issued to Fraden. The operating principle of an AFIR sensor can be illustrated by the following example. In a sensor's housing, there is a sensing element which contains a temperature detector (for instance, a thermistor) and a heater (a constant+resistor). The heater is thermally coupled with the detector. The sensing element is connected to an electronic circuit which measures the element's temperature through a temperature detector and provides electric current to the heater to elevate its temperature above ambient. The circuit maintains the element's temperature on a predetermined T.sub.s level which in many cases is above the highest temperature of a target. Thus, being warm, an AFIR sensing element becomes a source of infrared radiation whose net flux is guided toward a target. The magnitude of that flux relates to a temperature gradient between the known temperature T.sub.s and the unknown temperature T.sub.b of a target. Under the idealized conditions, according to law of conservation of energy, heat .PHI. radiated from the sensor toward the target must be equal to electric power P supplied to the resistive heater. The idealized conditions here mean that the only way the element may lose its thermal energy is by radiating it toward the target. Electric power provided to the heater can be expressed through value of the heater's resistance, R, and voltage V across it: ##EQU1##
Hence, combining equations (1) and (2), we can calculate the target temperature as ##EQU2##
It is seen that the calculated temperature of a target depends only of one variable which is the voltage V across the heating resistor. All other parts of equation (3) are either constant or predetermined. Further, if electronic circuit is efficient in maintaining T.sub.s level independent of T.sub.b, the element's temperature doesn't change upon exposure to a target and the AFIR sensor becomes not only accurate but fast as well. This can be accomplished with no stringent requirements to thermal capacity of a sensing element. In effect, an AFIR sensor is a direct and efficient converter of electrical power into thermally radiated power. Value of T.sub.s typically is selected in the range from 40.degree. to 100.degree. C. For medical applications, it is preferably near 50.degree. C.
In reality, an AFIR sensor doesn't operate under idealized conditions. Thermal energy dissipated by the heating resistor, goes not only toward the target, but to all components of a sensor's internal structure as well. Further, heat propagates from the element not only by means of radiation, but also through thermal conduction and gravitational convection of gas inside the sensor's housing. This makes use of formula (3) quite inaccurate, as it doesn't contain an additional variable whose influence becomes quite strong. This variable is temperature of the sensor's housing, or, in other words, the environmental (ambient) temperature. That is, a major difficulty in design and use of AFIR sensors relates to compensating for stray heat loss from the sensor system.
To compensate for undesirable thermal loss from a sensing element, several methods and system arrangements have been proposed. For example, see U.S. Pat. No. 4,854,730, and U.S. Pat. No. 5,054,936 issued to the present applicant. The teachings of these two patents are incorporated by reference as if restated herein in full. While these approaches have been successful in addressing some of the error inducing signals associated with AFIR sensors, there remains a need to enhance overall performance in signal isolation and reading accuracy. It was with this understanding of the prior art systems that the present invention was realized.